1. Field of the Disclosure
This disclosure relates generally to apparatus and methods for drilling wellbores for production of hydrocarbons.
2. Background of the Art
Wellbores (or boreholes) are drilled in earth formations to develop reservoirs for the production of hydrocarbons (oil and gas). It is often desirable to drill multiple wellbores in a formation in predetermined relationships to an existing well. For example, it is sometimes desirable to drill a number of closely spaced horizontal wellbores for recovery of hydrocarbons from a reservoir. A variety of methods, including seismic, magnetic and electromagnetic methods, have been used for drilling such multiple wellbores.
The magnetic methods generally fall into two categories. In the first category, referred to as active ranging, a source of AC magnetic field and a magnetic sensor are placed in different wells. The source may be a solenoid placed in a production wellbore or an electric current injected in the production well casing. The magnetic field produced by the current in the casing is measured in a drilling well spaced from the production wellbore. The active ranging approach can often offer a good accuracy of measurements, but requires access to a pre-existing wellbore.
In the second category, referred to as passive ranging techniques, access to a pre-existing well is not required for drilling additional wellbores. The passive ranging techniques normally utilize a relatively strong magnetism induced in the casing of the pre-existing well by the Earth's magnetic field. The measured signal due directly to the earth's magnetic field is a problem, limiting the accuracy of this measurement. Residual magnetism of the casing introduces additional noise in the measured signal.
The active magnetic ranging and seismic methods generally utilize sensors in a previously drilled wellbore and sources in the wellbore being drilled. The signals generated by the sources are received by the sensors and processed to give the location of the well being drilled relative to the preexisting well. Alternatively, the signal sources may be placed in the preexisting wellbore and the sensors in the wellbore being drilled. This requires instrumentation in both wellbores, synchronization of clocks in both wellbores and extensive use of telemetry resources.
The present disclosure provides apparatus and methods for drilling wellbores that address some of the above-noted issues.